U.S. Pat. No. 3,782,486 discloses a device for sensing and measuring a transient load such as one imposed by at least one wheel on an axle of a passing vehicle. The device comprises a low profile pad or mat comprising three electrode plates separated by a dielectric, elastomeric material. Capacitance is established between the central electrode and each of the outside electrodes. The electrodes are connected to a device for detecting variations in the electrode spacing owing to transient loads. Void areas are located in the body of the mat to provide for mat deformation proportional to the magnitude of the applied load. The patent describes the need for a portable weight sensing device having the characteristics of (a) portability, (b) simplicity, (c) ruggedness and reliability, (d) insensitivity to location of load and (e) cheapness. Disclosure of other capacitance weighing transducers is found in British patent specification 1,453,934 and Canadian patents 1,069,945; 1,013,779 and 901,028. All of these transducers have different types of elastomeric dielectric materials. All measure a load using the change in the capacitance of the transducer caused by compression of the dialectic.
Despite some claims to the contrary, capacitance transducers using elastomeric dielectric materials may not have a linear response and may have significant hysteresis under a variety of conditions and loads. Tests on such mats suggest that the cause is that diametric properties of the combination of expansion gaps (or air holes) and elastomeric material between the electrodes may vary in a non-linear manner under compression, that elasticity of the material may be temperature dependant and that there may be memory effects in elastomeric material when loads are applied in rapid succession.
It is an object of this invention to construct an improved capacitance transducer which does not rely upon compression of elastomeric dielectric materials to change the capacitance. Instead, the present invention employs a novel principle of bending adjacent electrode plates in synchronous opposition over a plurality of substantially rigid dielectric separators to create a plurality of areas of closer proximity between the plates to effect capacitance changes.